Remove the 'UnresolvedCallPattern' I stubbed out.

I talked to John about parsing patterns today, and because of the magnitude of name-lookup-dependent ambiguities between patterns and expressions, we agreed that at least for a first-pass implementation it makes sense to parse patterns as extensions of the expr grammar and charge name binding with distinguishing patterns from expressions. This gets us out of needing the concept of an "unresolved pattern", at least in the short term.

Swift SVN r5808

lib/Serialization/Serialization.cpp

@@ -506,9 +506,6 @@ void Serializer::writePattern(const Pattern *pattern) {
                                  addTypeRef(isa->getCastTypeLoc().getType()));
     break;
   }
-  case PatternKind::UnresolvedCall:
-    llvm_unreachable("serializing unresolved ast");
-
   case PatternKind::NominalType: {
     auto nom = cast<NominalTypePattern>(pattern);